More than 1 trillion becquerels of radioactive substances were released into the environment during debris-clearing work at the Fukushima No. 1 nuclear power plant last year, according to estimates by Tokyo Electric Power Co.

Although TEPCO's stricken facility is estimated to currently emit about 10 million becquerels per hour, the utility said that cleanup efforts at the plant’s No. 3 reactor on Aug. 19, 2013, resulted in the release of a maximum 1.1 trillion becquerels of radioactive materials over a period of four hours.

The figure was presented at a meeting of the Nuclear Regulation Authority on July 23.

“More careful countermeasures should have been taken,” said Toyoshi Fuketa, an NRA commissioner.

The estimates were made by monitoring results in downwind areas and also took into account that alarms for highly contaminated dust concentrations sounded for four hours.

The precise amount of radioactive substances that leaked out of the plant site remains unclear, TEPCO officials said.

The utility said the concentration levels were the highest during a 20-minute period, and it applied that figure for all of the four-hour cleanup effort. Therefore, TEPCO officials believe their final figure is probably an overestimate.

Since Aug. 19 last year, TEPCO has taken steps to prevent a recurrence, such as spraying anti-scattering agents not only before debris removal, but also after the operations. Dust concentration alarms have not sounded since the new precautions took effect, according to the company.

Tokyo Electric Power Co. submitted its plan to provide wide-ranging training programs for workers engaged in decommissioning of its crippled nuclear power plant in Fukuskima Prefecture following a string of accidents, some of them fatal.

Prosecutors again decided not to indict three former executives of Tokyo Electric Power Co. over the triple meltdown at the Fukushima No. 1 nuclear plant, essentially saying that the disaster was unpreventable.

Japan's nuclear watchdog gave the green light to the operator of the wrecked Fukushima nuclear power plant to discharge pumped up groundwater into the sea if radioactive substances in the water are within safety standards.